A capacitor made of a metal layer-deposited plastic film (hereinafter referred to as a “metallized film capacitor”) has conventionally found a wide range of applications. Particularly, a metallized film capacitor using a polypropylene (hereinafter referred to as PP) film has a wide range of applications, from a use for small electronic components represented by portable devices to a large-scale industrial use, e.g. drive motor control of a train and high-voltage phase advance. This is because a metallized film capacitor using a PP film has excellent electrical characteristics, e.g. small dielectric loss, high withstand voltage, and little change in a dielectric constant caused by changes in temperature and frequency.
FIG. 8 is a sectional view of a conventional metallized film capacitor using a PP film. The capacitor comprises two sheets of PP film 42 and metal-sprayed portions 34. The two sheets of PP film 42 have metal layer 41 deposited on one side thereof, and are overlaid and wound or laminated. The metal-sprayed portions are made by spraying metal from both end faces of the capacitor. As deposited metal layer 41, aluminum, zinc, and blends thereof have widely been used. Use of aluminum has a problem in that aluminum has a low adhesive strength with respect to metal-sprayed portions 34, and moreover, in application of voltages for an extended period of time, deterioration caused by oxidation of aluminum decreases capacitance. For this reason, zinc or a blend of zinc and aluminum has recently been used in many cases.
A widely employed structure of deposited metal layer 41 is a heavy edge structure shown in FIG. 8. A deposited metal layer in a capacitance-forming portion is thinner to improve a self-healing capability, and the deposited metal layer in a portion in contact with metal-sprayed portion 34 is thicker to enhance a bonding strength with metal-sprayed portion 34. The “self-healing capability” is a capability of the capacitor when local dielectric breakdown has occurred in a portion of a film, whereby evaporation and scatter of the deposited metal layer in the vicinity of the portion interrupts current.
With a conventional structure, two belts of PP film each having a deposited metal layer on one side thereof are used. Thus, each belt requires a vacuum deposition step and this necessitates a large number of man-hours.
If a metal layer can be deposited onto both sides of a PP film (hereinafter referred to as a “both-side metallized PP film”) by one vacuum deposition step, and laminated over a non-metallized polypropylene film (hereinafter referred to as a “PP film for combination”) to form a structure (see FIG. 7), the vacuum deposition step can be reduced to one half.
However, this method has a problem. When both sides of a film are metallized, the film needs to be wound once as product roll 3 in vacuum deposition machine 1 (see FIG. 1). At this time, deposited metal layers on both sides make contact with each other. The PP film has low wettability, and thus has low adhesive strength with respect to a deposited metal layer. In addition, the product roll is tightly wound (i.e. a winding stress accumulating inside of the product roll is strong). Thus, deposited metal layers on both sides adhere to each other (hereinafter referred to as “blocking”) and are peeled by each other, when the film is drawn out of the product roll in subsequent steps, i.e. steps of slitting the metallized film and winding the film into a capacitor. As disclosed in U.S. Pat. No. 3,895,129, it is known that blocking more often occurs with a deposited metal layer of zinc than aluminum.
When a capacitor is produced by a both-side metallized PP film where blocking has developed, deposited metal layers on both sides are peeled by each other in the above-mentioned slitting and winding steps. This peeling deteriorates a function of capacitor electrodes and increases tan δ (dielectric loss tangent). Therefore, solving this blocking problem is essential to a both-side metallized PP film for a capacitor.
With regard to this point, a manufacturing method that decreases blocking has been proposed. For example, disclosed in the U.S. Pat. No. 3,895,129, as a manufacturing method of depositing zinc on both sides of a PP film, is a method of spraying air onto one of deposited surfaces to oxidize this deposited metal layer and then winding the film into a product roll.
Described in the Japanese Patent Application Non-examined Publication No. H07-62238, as a manufacturing method of depositing zinc, zinc alloys, or other metal layers on both sides of a synthetic resin film, is a manufacturing method of injecting oxidizing gases such as air and oxygen into a film winding room, that is in a vacuum chamber and separated from a vacuum deposition room by a partition. In other words, disclosed are the following steps: depositing a metal layer onto one side of a PP film; spraying an oxidizing gas onto a deposited surface to oxidize the deposited layer; depositing a metal layer onto an opposite side; spraying an oxidizing gas onto this deposited surface; and winding the film.
However, for the manufacturing methods disclosed in U.S. Pat. No. 3,895,129 and Japanese Patent Application Non-Examined Publication No. H07-62238, the blocking problem in a both-side metallized PP film using zinc or a blend of zinc and aluminum has not yet been solved.
In the manufacturing methods disclosed in U.S. Pat. No. 3,895,129 and Japanese Patent Application Non-Examined Publication No. H07-62238, air or oxidizing gas is sprayed onto a deposited surface of a PP film running at high speeds. Because running speeds of a PP film in a vacuum deposition process typically range from 300 m to 1,000 m per minute, the PP film is exposed to the air or oxidizing gas only for an extremely short period of time such as at most one hundredth of a second. Such a short exposure cannot form a sufficiently thick oxide layer on a deposited film of zinc or a blend of zinc and aluminum, and thus has not led to a solution of blocking. There are still the following problems.
Especially with a both-side metallized PP film for a capacitor, continuous deposition is generally performed on a long belt of PP film more than 10,000 m in length. For a product roll of such a long belt, winding stress from outside accumulates in inner winding portions (in the vicinity of a core) thereof. This causes blocking when the oxide film is insufficiently thick. When a capacitor is produced using such a both-side metallized PP film, tan δ of the capacitor increases as mentioned above.
Blocking is a phenomenon caused by adhesion of deposited layers on front and back surfaces of a PP film. Therefore, after the film has been stored for an extended period of time in a state of a product roll, an insufficiently thick oxide film again causes blocking and increases tan δ in a production of a capacitor. This is another problem.
In addition, when a metallized raw material roll is affected by moisture during long-term storage or transportation thereof, blocking may develop secondarily. Development of blocking caused by moisture occurs more frequently when a deposited metal layer is zinc or a blend of zinc and aluminum. This is because zinc is changed to oxide and hydroxide by moisture more easily.
Especially when a PP film is used as the film, deposited metal layers on front and back surfaces of the film adhered to each other by moisture easily peel off from the film. This is because the PP film has lower adhesive strength with a deposited metal layer than other film materials.
When a capacitor is produced using a both-side metallized film in which blocking has developed in this manner, wrinkles occurring during slitting and winding steps decrease a withstand voltage of the capacitor, or peeling of deposited layers, or oxidation or hydroxylation, of this deposited metal increases an electrode resistance to increase tan δ.
For this reason, when the both-side metallized film is stored or transported in a state of a raw material roll, the film must be kept at humidity lower than a one-side metallized film, or hermetically sealed in a package with a desiccant.
It is an object of the present invention to provide a method of manufacturing an excellent both-side metallized PP film having zinc, or a blend of zinc and aluminum, deposited onto both sides of a roll of a long PP film that is free from blocking even when the roll is stored for an extended period of time or affected by moisture. It is another object of the present invention to provide an excellent capacitor that requires a small number of man-hours and has no increase in tan δ caused by blocking.